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10-10-2016

Limit inrush current in high-power applications

STMicroelectronics » TYN610

JB Castro-Miguens, C Castro-Miguens

EDN

Protect circuits from high current until a capacitor charges

A high-power offline supply is nothing more than a half- or full-bridge dc/dc converter. Rectifying the ac line yields a dc voltage that feeds the converter. At power-supply turn-on, the bulk capacitor of the uncontrolled rectifier is completely discharged. It results in a huge charging current for a high instantaneous line voltage because the discharged bulk capacitor temporarily short-circuits the diodes of the rectifier stage. This can result in a large charging current for a high instantaneous line voltage because the discharged capacitor temporarily short-circuits the power supply’s diode bridge. The high inrush current can trigger a mains circuit breaker, burn a fuse, or even destroy a power supply’s rectifier diodes unless you take precautions. The circuit in Figure 1 limits the inrush current.

Limit inrush current in high-power applications
Figure 1. A thyristor and a MOSFET control current to bulk capacitor CO.
This circuit limits the inrush current.

At turn-on, if the instantaneous rectified ac-line voltage, VACR, is greater than approximately 10 V, Point A in Figure 2, MOSFET Q2 turns on, forcing thyristor Q1 off. In this situation, a little current flows through R1 and Q2, injecting a small charge into bulk capacitor CO, Path A to B in Figure 2.

Limit inrush current in high-power applications
Figure 2. If VACR is greater than approximately 10 V, MOSFET Q2 turns on;
current flows through R1 and Q2, injecting a small charge
into bulk capacitor CO.

When VACR – VO ≤ 8 V or so, where VO is the output voltage, Q2 is off, letting Q1 conduct. In this situation, the bulk capacitor receives the necessary charge through Q1, Path B to C in Figure 2, to match VO to VACR. After this point, VACR falls below VO, and the bulk capacitor alone must support any power the dc/dc converter demands until VACR – VO ≥ 5 V or so, Path C to D in Figure 2. At Point D, VACR – VO ≈ 5 V and thyristor Q1 triggers, which conducts the capacitor’s charge current and the current the dc/dc converter demands until VACR matches the sinusoidal peak at Point E.

When VACR falls, thyristor Q1 cuts off, and the bulk capacitor alone feeds the dc/dc converter. The thyristor conducts again when VACR matches VO to the sinusoidal peak. This process then repeats. Use a nonsensitive gate thyristor with a breakdown voltage of at least 400 V for an ac voltage of 220 V rms (root mean square) and with twice the rms-current rating of the rectifier diodes.

This circuit uses a TYN610 thyristor. You can calculate the value of R1 using

where VGT is the minimum gate-cathode voltage necessary to produce the gate-trigger current for Q1 and IGT–20° is the minimum gate current to trigger Q1 down to –20 °C. The NTD4815NHG MOSFET is suitable for this circuit. A MOSFET with a different threshold voltage may require different values for R2 and R3.

Materials on the topic

  1. Datasheet STMicroelectronics TYN610
  2. Datasheet ON Semiconductor NTD4815NHG

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  • The unusual decision. But perhaps it is operational. Check one must ... And if we compare this with the scheme that is used in welding inverter? There pitalovo first fed through a resistor, then it closes relay contacts, and full power is supplied. In the scheme of welding inverters plus it is the simplicity, reliability relay compared to triac basking in this scheme. The disadvantage in welding, I think - too much time has been set for the operation of the relay. Then the time to expose itself, depending on the load. Only thyristor here I do not like.

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